Something that helps us a lot to keep on track is feedback from people like yourself. I hope you're trying it out already, so that if you identify potential issues we can solve them in the current sprint, rather than being a blocker for you after it's released.

Version 4.2.1 is quite old. May I suggest that, even if you don't jump to 5.8.x yet, you already get your application updated to some recent 5.x version? That will make it much easier for you to later update to 5.8+ as there are other migration steps which will already have been taken, for example upgrading the Hibernate ORM version.

N.B. Even if we release version 5.8 on time we still consider the Elasticsearch integration experimental, as the current API is very Lucene-centric. The internals should be stable and safe to use, but we plan to work on a major version 6.0 next and make the API "Lucene agnostic" so that people using Elasticsearch won't have to code with Lucene APIs in the way.